It is not too difficult for Liverpool to beat Southampton who are currently languishing at the bottom of the standings, but Arne Slot will not take any risks in this game because he must keep his core players fit to face PSG in the second leg of the UCL. Rotation will be carried out in several positions to prevent their mainstay players from getting tired or in the worst case scenario getting injured, but Liverpool must still prioritize their goal of getting three points to prevent Arsenal from closing the gap on them.
The best situation is trying to do some rotation of players to maintain fitness in preparation for facing PSG in the second leg. But if the rotation is not done properly, it will slightly affect the pattern of the game and it is possible that it will end in a draw or defeat. The Premier League is different because even though the team is at the bottom of the standings, if there is a small mistake they can punish the top team.
Arne Slot must be careful if he wants to rotate because after all, victory is something they must get in even easy matches because unexpected results can happen if Arne Slot makes the wrong rotation of players for the remaining matches ahead.
Liverpool have the advantage of playing at home and their opponent is the team that is currently at the bottom of the standings, besides that they also have a fairly wide gap with Arsenal, so this is the right time to rotate the squad. It is not meant to belittle, but this choice had to be taken because the Champions League requires more attention as I'm pretty sure that PSG will go all out in the second leg. Liverpool should still be able to win over Southampton even if they don't play with a full team.
In addition, Liverpool also has a slight advantage in this week because Arsenal will play as guests against Man United, so Arsenal could fail to get optimal results.
You're right, southempton always bring their best performance more especially when they are playing with bigger teams. on the second thought southempton is in the bottom of the table just about going for relegation this season, with this I think the match is going to be a little bit tough for liverpool. As liverpool will also want to reserve those thier key players ahead of thier upcoming match against psg in the champions league, I think that is thier major concern for now. But regardless to that I still have this strong believe that liverpool will gain victory over southempton.